According to royal correspondent Richard Palmer, the duchess's recent slew of secret events are due to the fact that many of these appearances have taken place in central London, meaning crowd control could pose a problem to traffic and security.
Since, in all recent instances, the reason for Kate's visit wasn't to meet crowds, but simply further the work of the charities she supports and patronages she represents, it's not deemed royally necessary to announce the engagement ahead of time.
"The palace generally embargoes engagements until arrival when there might be big crowds around," Palmer wrote on Twitter. For example, "in central London or at tourist attractions where the point of the visit is not to meet those crowds."
Ahh, makes a lot of sense. But is it just because she's the Duchess of Cambridge? What about the other royals?
Palmer went on to write, "It’s a security issue in a way: they’d rather not create a distraction that causes congestion and attracts randoms. It’s not just Kate. Sometimes Charles and Camilla’s engagements in central London are embargoed, for example."
He also posited a theory for why Kate's been stepping out in central London so often, as opposed to other members of the royal family whose official engagements take place outside London and all over the country...and the real reason is something any parent can relate to.
Middleton, Duchess of Cambridge, Princess of the United Kingdom, Countess of Strathearn, Lady Carrickfergus and Dame Grand Cross of the Royal Victorian Order, etc., etc., has to fit her royal engagements "around her children’s school hours," Palmer wrote.
Five minutes late for George and Charlotte's school pickup? Kate wouldn't dare.